Sulfuric acid (H2SO4) and oleum (which is a solution of sulfur trioxide in concentrated sulfuric acid namely SO3·H2SO4) are widely used as sulfonating agents for the sulfonation of aromatic compounds. Sulfuric acid (H2SO4) and oleum always need to be used in large excess as water is formed in the sulfonating reaction thereby diluting the oleum and/or sulfuric acid. This has the disadvantage of leaving large quantities of unreacted sulfuric acid. This waste acid must be separated from the reaction mixture and subsequently disposed of. This acid is difficult to dispose of, either as the free acid or in the form of soluble or insoluble sulfates, particularly now when effluent requirements are becoming more stringent.
Alternatively, sulfur trioxide itself in gaseous or liquid form, has also been used for the sulfonation of aromatic compounds. Sulfur trioxide reacts instantaneously with aromatic compounds, and it is not necessary to use a substantial excess to realize complete sulfonation. There is, therefore, no need for the reaction product to be contaminated with substantial quantities of excess sulfonating agent. Sulfur trioxide itself can notably be prepared from oleum. For example, distillation of oleum can generate pure SO3 vapour which can be used as such or can be further condensed to form pure liquid SO3. However, pure gaseous or liquid sulfur trioxide is very highly reactive and its reactions with aromatic compounds are extremely exothermic and difficult to control and undesirable side reactions might occur. It is known that in order to moderate and control the reactions of pure gaseous or liquid SO3 with aromatic compounds, SO3 has been used in the presence of inert diluents. For example, air/SO3 sulfonation processes are widely used in the surfactants and detergent industry for the sulfonation of long chain and/or high molecular weight organics, such as for example low volatile long chain alkyl benzenes, which are characterized by having high viscosity and high flash points.
The diluted, gaseous SO3 mixture is still a very aggressive/reactive material.
For this reason, it is known that the use of the air/SO3 sulfonation process for the sulfonation of more volatile compounds such as notably toluene, xylene and other lower alkyl benzenes are problematic and said process is in general less suitable for the sulfonation of volatile aromatic compounds.
Thus, there is still a considerable need for a process for sulfonating volatile halobenzene compounds with sulfur trioxide (SO3) in a controllable manner, which avoids the use of liquid SO3 which entails some serious concern for its security, transportation, and rigorous storage requirements imposed by the hazardous nature of liquid SO3, capable of providing para-substituted halobenzene sulfonic acid compounds in high yield and 4,4′-dihalodiphenyl sulfone, both in high purity and high selectivity, to minimize of any formation of oversulfonated products as a result of undesirable side reactions and avoid unconverted reactants, yet without any significant problem of disposal of excess sulphuric acid, much less hazardous for the environment and simultaneously in achieving significant cost savings.